H.sub.2 --O.sub.2 fuel cells use hydrogen as a fuel and oxygen (as air) as an oxidant. The hydrogen used in the fuel cell can be derived from the reformation of methanol or other organics (e.g. hydrocarbons). For example, in the methanol reformation process, methanol and water (as steam) are ideally reacted in a catalytic reactor (a.k.a. "reformer") to generate a reformate gas comprising hydrogen and carbon dioxide according to the reaction: EQU CH.sub.3 OH+H.sub.2 O.fwdarw.CO.sub.2 +3H.sub.2
One such reformer is described in U.S. Pat. No. 4,650,727 to Vanderborgh. Unfortunately, the reformate exiting the reformer also contains undesirably high concentrations of carbon monoxide most of which must be removed to prevent poisoning of the catalyst of the fuel cell's anode. In this regard, carbon monoxide (i.e., about 1-3 mole %) contained in the H.sub.2 -rich reformate/effluent exiting the reformer must be reduced to very low nontoxic concentrations (i.e., less than about 20 ppm) to avoid poisoning of the anode.
It is known that the carbon monoxide, CO, level of the reformate/effluent exiting a methanol reformer can be reduced by utilizing a so-call "shift" reaction wherein water (i.e. steam) is added to the methanol reformate/effluent exiting the reformer, in the presence of a suitable catalyst. This lowers the carbon monoxide content of the reformate according to the following ideal shift reaction: EQU CO+H.sub.2 O.fwdarw.CO.sub.2 +H.sub.2
Some (i.e., about 0.5 mole % or more) CO still survives the shift reaction, and any residual methanol in the reformate is converted to carbon dioxide and hydrogen in the shift reactor. Hence, shift reactor effluent comprises hydrogen, carbon dioxide, water and carbon monoxide.
The shift reaction is not enough to reduce the CO content of the reformate enough (i.e., to below about 20 ppm). Therefore, it is necessary to further remove carbon monoxide from the hydrogen-rich reformate stream exiting the shift reactor, and prior to supplying it to the fuel cell. It is known to further reduce the CO content of H.sub.2 -rich reformate exiting the shift reactor by a so-called "PrOx" (i.e., preferential oxidation) reaction effected in a suitable PrOx reactor operated at temperatures which promote the preferential oxidation of the CO by air in the presence of the H.sub.2, but without consuming/oxidizing substantial quantities of the H.sub.2 or triggering the so-called "reverse water gas shift" (RWGS) reaction. The PrOx and RWGS reactions are as follows: EQU CO+1/20.sub.2 .fwdarw.CO.sub.2 (PrOx) EQU CO.sub.2 +H.sub.2 .fwdarw.H.sub.2 O+CO(RWGS)
The PrOx process is described in a paper entitled "Methanol Fuel Processing for Low Temperature Fuel Cells" published in the Program and Abstracts of the 1988 Fuel Cell Seminar, Oct. 23-26, 1988, Long Beach, Calif., and in Vanderborgh et al U.S. Pat. No. 5,271,916, inter alia.
Desirably, the O.sub.2 required for the PrOx reaction will be about two times the stoichiometric amount required to react the CO in the reformate If the amount of O.sub.2 exceeds about two times the stoichiometric amount needed, excessive consumption of H.sub.2 results. On the other hand, if the amount of O.sub.2 is substantially less than about two times the stioichiometric amount needed, insufficient CO oxidation will occur and there is greater potential for the RWGS reaction to occur. Accordingly in practice, many practitioners use about 4 or more times the stoichiometric amount of O.sub.2 than is theoretically required to react with the CO.
PrOx reactors may be either (1) adiabatic (i.e. where the temperature of the reactor is allowed to rise during oxidation of the CO) or (2) isothermal (i.e. where the temperature the reactor is maintained substantially constant during oxidation of the CO). The adiabatic PrOx process is sometimes effected via a number of sequential stages, which progressively reduce the CO content in stages, and requires careful temperature control, because if the temperature rises too much, the RWGS reaction can occur which counter productively produces more CO. The isothermal process can effect the same CO reduction as the adiabatic process, but in fewer stages and without concern for the RWGS reaction if (1) the reactor temperature can be kept low enough, and (2) O.sub.2 depletion near the end of the reactor can be avoided.
One known isothermal reactor is essentially a catalyzed heat exchanger having a thermally conductive barrier or wall that separates the heat exchanger into (1) a first channel through which the H.sub.2 -rich gas to be decontaminated (i.e. CO removed) passes, and (2) a second channel through which a coolant flows to maintain the temperature of the reactor substantially constant within a defined working range. The barrier wall has a catalyzed first surface confronting the first channel for promoting the CO+O.sub.2 reaction, and an uncatalyzed second surface confronting the second channel for contacting the coolant therein to extract heat from the catalyzed first surface through the barrier. The catalyzed surfaces of adjacent barriers oppose each other, and are closely spaced from each other, so as to define a narrow first channel through which the H.sub.2 -rich gas moves under substantially laminar flow conditions. Unfortunately, under such flow conditions, only the outer layers of the gas stream moving through the first channel contact the catalyst for reacting the CO and O.sub.2 therein. The central layers of the gas stream, midway between the opposed catalyzed surfaces, can actually pass through the first channel unreacted by the catalyst, and hence retain much of their CO and O.sub.2 unreacted. Moreover, hot or cold spots, flow maldistribution, or nonuniformities within the reactor (e.g. non-uniform catalyst loadings), can cause different rates of reaction to occur at different sites within the heat exchanger, which in turn, can cause some of the CO and O.sub.2 in the gas stream not to react. The net effect is that considerable CO and O.sub.2 can pass unreacted through the reactor unless the reactor is made very large.
The present invention overcomes the aforesaid problem by providing an efficient, multi-stage reactor that periodically homogenizes the gas being treated en route through the reactor so as to substantially uniformly distribute the CO and O.sub.2 in the gas between different reaction stages of the reactor. The outer layers of the gas stream contacting the catalyst downstream of homogenization are hence enriched with CO and O.sub.2 for more effective reaction on the downstream catalyst. Moreover, by introducing some of the air into the gas stream at the homogenization site, it is possible to reduce the excess O.sub.2 needed for the reaction as well as reduce the incidence of the RWGS reaction near the end of the reactor.